PURPOSE: To realize the on-line measurement of the lateral bend of the steel plate by measuring the angle formed by the shift amount in both the carriage line direction of the steel plate and its right- angled direction and the straight carriage line connecting the measuring points and then calculating the bend amount.

CONSTITUTION: When measuring the bend from point A on side edge 5 of the steel plate to point B, E1 is set for the point of the side edge where point A passes through point K in lapse of time t1 after passing installation position K of measuring roller 6. And thus shift amount l1 of the steel plate in the carriage direction is measured by calculator 16 and based on velocity v of the steel plate and passing time t1; while shift amount w1 vertical to the carriage direction is measured by calculator 15 respectively. At the same time, angle α formed by the straight line connecting points A and B and the line of the carriage direction can be calculated by obtaining BC of triangle ABC. Accordingly, distance m1 orthogonal to straight line AB from point E1 can be calculated by Eq. 1, and the miximum value between A and B of the distance means the bend amount between A and B. In such way, the bend amount of the steel plate can be measured by the on-line method.